Mass spectrometry (MS) and ion mobility spectrometry are analytical tools used for qualitative and quantitative elemental or molecular analysis of samples to measure the mass and size of ionized particles respectively.
In an effort to identify the nature of certain molecules in a given sample, a mass spectrometer may be used. Mass spectrometers require molecules or molecular species present in a sample to form gas phase ions. The ionized molecules, or analyte ions or simply analyte species, can consequently be directed by electrical fields to a mass analyzer device to separate in space and/or time due to their relative difference in mass-to-charge ratios. In turn, a separate detector device in the mass spectrometer is able to generate a mass spectrum. An ion mobility spectrometer also requires molecules or molecular species to form gas phase ions, which can consequently be separated in space and/or time due to their relative difference in size and/or charge. In turn, a separate detector device in an ion mobility spectrometer is able to generate a mobility spectrum. It may also be preferable to connect an ion mobility spectrometer to a mass spectrometer in tandem to obtain an ion mobility spectrum and a mass spectrum sequentially or simultaneously.
A mass spectrum is useful to derive information about the mass-to-charge ratios and in some cases the quantities of the various analyte ions of molecules or molecular species that make up the sample. A mass spectrum is also useful to derive information about the mass-to-charge ratios of fragment species comprising the analyte ions.
Similarly, an ion mobility spectrum provides information about the collisional cross-section and the charge state of analyte species. From the cross-section information one can infer a geometrical configuration, identify molecular conformation and or the charge state of the various analyte species. Preferably an ion mobility based spectrometer, for example a differential mobility spectrometer, is incorporated into a mass spectrometer to perform both forms of detection from which to draw inferences about complex samples of molecular species under analysis.
A prevalent configuration of a mass spectrometer uses an ionization source to introduce ions entrained in a gas in a high pressure environment (e.g., atmospheric pressure). Preferably ionization is set up to occur in the gas phase near the inlet opening of a first of a series of vacuum compartments. The first vacuum compartment in the series of vacuum compartments is the fore vacuum compartment. Each vacuum compartment maybe further divided into sub-compartments operated at substantially the same pressure.
The series of vacuum compartments, which form a mass spectrometer, are operable to receive gas phase ions entrained in a flow of gas at the fore vacuum compartment and carefully designed to direct ions by ion optical means, for example by using electrical fields, into subsequent vacuum compartments of lower pressure. The aim in doing this is to efficiently transport the entire initial ion population from a high pressure region, for example the ionization source, to the high vacuum compartment enclosing the mass analyzer, whilst the initial gas load is progressively removed by means of vacuum pumps.
A critical part of a mass spectrometer is the design of the fore vacuum region where significant ion loses are known to occur near the inlet opening or the distal end of this region determined by a second pressure-limiting aperture defining the entrance to the second vacuum compartment. Yet another critical aspect of the performance of a mass spectrometer in terms of sensitivity and overall ion transmission is the gas dynamics design of the fore vacuum region. It is well appreciated that sensitivity is directly related to the properties of the gas flow, more precisely the formation of the free jet expansion flow and associated ion optical means arranged to focus ions under these conditions.
Consequently, it is desirable to be able to achieve improvements in the aerodynamic design of the fore vacuum region of a mass spectrometer to improve ion transmission and also achieve appropriate gas flow fields for performing ion separation based on ion mobility.